1. Field of the Invention
This invention relates generally to resonant-element instruments that measure a physical variable representative of an industrial process condition. More particularly, this invention relates to an apparatus for exciting the resonant element into vibratory motion.
2. Description of the Prior Art
It has been well known for many years that the resonant vibrational frequency of a taut wire is a function of the tension being applied to that wire. It was also long recognized that if an industrial process condition is converted to a force and applied to an instrumentality such as a diaphragm or bellows which holds the wire in tension, then a measurement signal representing the magnitude of the process condition can be produced by causing the wire to vibrate and using the detected frequency of vibration to generate the measurement signal.
In some of the prior art instruments, the vibratory wire was formed of a magnetic material. That wire having wire ends clamped in place was made to vibrate via the influence of an alternating magnetic field produced by an electromagnetic drive coil. Detection of the frequency of wire vibration was usually by a pickup coil that was inductively or capacitively coupled to the wire. The pickup coil produced a voltage signal having variations that corresponded to the frequency of wire vibrations. As is well known, both drive and pickup coils were disposed closely adjacent the wire not only for positioning the wire in a region of high magnetic field intensity but also for obtaining a relatively large magnitude voltage signal from the pickup coil.
In other prior art instruments, the wire was made of an electrically conductive material and was immersed in a strong magnetic field produced by permanent magnets. When an oscillating current flowed through the wire, the magnetic field interacted with that current and caused the wire to be displaced back and forth from its rest position. By advantageously controlling the frequency of the oscillating current, the wire displacements were sustained at the resonant frequency of the wire. An electronic oscillator that develops the alternating current for maintaining wire vibrations at the resonance frequency of the taut wire is known and is disclosed, for example, in U.S. Pat. No. 4,118,977 issued to Olsen et al entitled, "Electrical Signal Transmitter for Vibrating-Wire Sensor."
Although the prior art instruments described above have performance abilities suited for their intended purposes, these instruments have limitations when used in certain applications. Specifically, having electrical currents in an hazardous environment may dictate that expensive casings are required to contain the electrically active devices so as to prevent disasters such as an explosion. There is also the problem of sealing the electrical and signal leads that must be routed through each of the casings for connection to the drive and pickup coils or to the electrically-conductive wire of the respective instruments described above.
Furthermore, in the instruments of the type having permanent magnets, materials such as samarium cobalt are often used for producing a strong and intense magnetic field about the vibrating wire. However, these materials are expensive and their costs are a substantial part of the costs for producing this type of instrument. Additionally, the permanent magnets are usually joined to pole pieces which have specially-configured faces that concentrate the magentic field into a small region about the wire. But production costs are again increased because of not only the added costs for making these pole pieces but also the additional assembly steps of aligning the pole pieces to have a narrow gap between one another and locating those faces closely adjacent the wire.
Therefore, in view of the above, there is a need for an improved resonant element instrument which utilize neither coils nor magnets for producing vibratory motion.